Aluminium and heavy metal induced organic acid exudation of forest tree roots and the role of ectomycorrhizas

Poplar roots in the treatment solution. Foto: Ruijun Qin (WSL).

Objectives

Large parts of European forests grow on acidic soils. In the last decades, however, inputs of acidifying substances to forest ecosystems have increased and have led to an acceleration of the acidification processes on sensitive sites. In addition, within the last years, depositions of heavy metals have increased. Thus, Al and heavy metals potentially can reach phytotoxic concentrations in forest soils.
Metal toxicity is a major constraint affecting root growth in a number of natural and managed ecosystems. Although some metals such as Cu and Zn are essential nutrients for plants, when present in excess, these, and nonessential metals such as Al, Cd, Pb, and Hg can be toxic. Plants, like other organisms, possess a large range of mechanisms that detoxify metals and maintain intracellular metal homeostasis. One important mechanism in crop plants for metal tolerance, and in particular for Al tolerance, is the exudation of organic acids as efficient chelators. It has only recently been demonstrated that citrate or oxalate are also exuded under enhanced concentrations of Cu or Pb.

Aim

This project has the aim to investigate whether tree roots, similarly to many crop plants, exude organic acids at enhanced concentrations of Al or heavy metals. The following research questions should be answered:

• Which organic acids are exuded by tree roots after Al or heavy metal exposure?
• Which are the relevant concentrations of Al or heavy metals for organic acid exudation?
• Are the organic acids exuded by the roots specific for the tree species?
• What is the role of ectomycorrhizal fungi in organic acid exudation and in the heavy metal tolerance?
• Do root exudation patterns alter rhizosphere soil solution chemistry and thus the solubility of Al and heavy metals?

Methods

• Poplar will be tested for root exudation.
• Exuded organic acids (citrate, malate, oxalate) will be analysed with capillary electrophoresis (Heim et al. 2001).
• The metals will be quantified with ICP-MS according to Brunner and Frey (2000) and localized in plant tissues and compartments with SEM-EDX or STEM-EDX according to Brunner and Frey (2000) and Frey et al. (2000). 
• Rhizoboxes (according to Dieffenbach et al. 1997, Wenzel et al. 2001) will be used to see if induced root exudation patterns can alter the rhizosphere soil solution chemistry.

Relevance for extension

The knowledge of the mechanism of Al and heavy metal tolerance by tree roots would help to understand the relevant prossesses in the forest soils. Knowing these mechanisms is an important prerequisite to assess the risk of increasing inputs of acidifying pollutants to forest ecosystems and thus to an acceleration of the acidification processes in forest soils on sensitive sites.

Finances

This project is financially supportet by the State Secretariat for Education and Research, and by COST 631 'Understanding and Modeling of Plant-Soil Interactions in the Rhizosphere Environment' (UMPIRE) 

Recent publications

Qin, R., Hirano, Y., Brunner, I. 2007. Exudation of organic acid anions from poplar roots after exposure to Al, Cu and Zn. Tree Physiology 27, 313-320.